PROJECT SUMMARY In spite of sophisticated checkpoints, repair mechanisms and other safeguards, cells occasionally enter and exit metaphase with damaged DNA. While much is known about the response of the cell to DNA damage during interphase, the response of a cell entering and exiting metaphase with damaged DNA is largely unexplored. Of particular interest is the consequence of entering and exiting metaphase with unrepaired double-strand breaks (DSB). DSB's are especially troublesome, because they produce chromosome fragments lacking telomeres, as well as fragments lacking centromeres (acentrics). Although acentrics chromosome fragments were described over a century ago and are a common feature of cancer cells, surprisingly little is known about their behavior and fate during mitosis, largely because it has been difficult to generate acentrics that can be analyzed in living cells. To directly address this issue, my lab has developed a system in Drosophila neuroblasts to efficiently generate DSBs at a single defined region at the base of the X chromosome (through induction of I-CreI endonuclease) and follow the behavior of the resulting acentric fragment through high resolution live microscopy. These studies reveal that in spite of lacking a kinetochore, acentrics exhibit highly delayed, but ultimately successful congression, sister chromosome separation, anaphase poleward segregation and incorporation into daughter telophase nuclei. We also discovered that the late segregating acentrics are accompanied by a series of cellular adaptations that facilitate its segregation and inclusion in daughter nuclei. These include an increase in cell and spindle length, expansion of the cytokinetic contractile apparatus, and delayed in initiation and completion of nuclear envelope reformation at telophase. We view congression and segregation of acentric chromosomes into telophase daughter nuclei as a mechanism of last resort for maintaining genomic integrity when the eukaryotic cell enters anaphase with unrepaired DSBs. Our goal is to identify structural and regulatory mechanisms driving acentric segregation and the accompanying cellular adaptations. To achieve this, we propose three specific aims to define the mechanisms that mediate: 1) acentric sister chromatid congression, 2) poleward segregation of the acentrics, 3) the cellular adaptations to the presence of an acentric during anaphase/telophase. It is anticipated that these studies will reveal unsuspected novel mechanisms operating during anaphase and telophase that maintain the integrity of the eukaryotic genome.